the agricultural benefits of those periodic floods are offset by damage to homes and cities, and, in some cases, to the people who inhabit them. the edges of flood plains are marked by levees- ridges of sediment left atop river banks by floods. once formed, levees serve as natural barriers confining rivers during periods of ordinary flow. they may even protect low-lying areas from flooding if the level of a river isn't too high. for this reason, artificial levees designed to contain a river during flood stages are often built. but artificial levees can themselves create problems. by confining the river to a narrow channel, levees accelerate the build-up of sediment, raising the river bed higher and higher. and levees can provide a false sense of security. if a river overtops its levees to flood the surrounding land, the levees can actually prolong flooding by preventing water from draining back into the river. most people don't appreciate the fact that the flood plain is a part of the stream itself. the flood plain is where rivers store discharge during periods of high flows and also